Lipid and Phase Specific Interaction of Alpha Toxin from S.aureus with Lipid Membranes

Abstract

The pore forming alpha toxin from S.aureus is secreted by the bacterium as a monomer and assembles up to heptameric oligomers on cellular und artificial membranes, to yield a transmembrane pore. Interaction with artificial lipid membranes requires the presence of a lipid with a phosphocholine headgroup, namely sphingomyelin or phosphatidylcholine. Both represent the major fraction of lipids in the outer plasma membrane: saturated sphingomyelin preferring the liquid ordered phase, and unsaturated phosphatidylcholine with a preference for the liquid ordered phase. Employing liposomes we could show that alpha toxin has a higher overall affinity for sphingomyelin compared to phosphatidylcholine if the lipids are in the same phase. Preferred binding to sphingomyelin usually is interpreted as a preference for rafts or domains in the liquid ordered phase, especially if also binding is increased in presence of cholesterol as is also the case for alpha toxin. However, this is not necessarily the case. Alpha toxin preferentially interacts with lipids in the liquid disordered phase as shown by fluorescence spectroscopy and microscopy. Since sphingomyelin is also present in the liquid disordered phase, albeit to a smaller extent compared to the liquid ordered phase, binding could occur both to sphingomyelin or phosphatidylcholine. A further complexity is added by the observation charged lipids further increase alpha toxin binding. Thus one can expect that the partition coefficient of alpha toxin between liquid ordered and liquid disordered phase is critically dependent on the detailed composition, and might very well be shifted by presence of proteins as is the case in the cellular membranes, without the need of any specific interaction between alpha toxin and the protein. Granted by the DFG (SFB 490)